A continuing problem exists in the field of protective coatings wherein zinc and cadmium are applied to a substrate such as a steel strip or article, to protect it against corrosion. The zinc or cadmium protective coating itself will corrode in time to give a white or whitish deposit commonly known as "Storage Stain" or "White Rust". It is common therefore to apply a thin protective coating to the zinc or cadmium surface which usually consists of a chromium compound or compounds and is known as a "Conversion" coating.
The conversion coating is usually formed by applying a suitable aqueous solution containing chromium to the zinc or cadmium surface, the aqueous solution reacting with the surface to form a thin gel-like film of complex chromates. The film is substantially non-porous to moisture, and also is noncrystalline, so that it provides a good paint bond without absorption of the paint onto the surface. Examples of processes for producing such a coating are disclosed in U. S. Pat. Nos. 2,035,380, issued Mar. 24, 1936 to Wilhelm, and 3,457,124, issued Jul. 22, 1969 to Bohman.
The process solutions disclosed in these patents include specified amounts selected from sodium dichromate/bichromate, chromic acid, sulfamic acid, ammonium sulfate, boric acid, sodium silicofluoride and nitric acid. One of the more widely used metallic conversion systems for aqueous treating of selected metals and/or their alloys (e.g., zinc, cadmium . . . applicable others) consists of incorporating into the system small amounts of a molecularly dehydrated alkali metal polyphosphate in combination with a soluble inorganic chromate such as sodium dichromate. This is known in the art as the "Polychrome" treatment, which while very effective in controlling applicable metallic corrosion resistance, is subject to many disadvantages.
One o f the major disadvantages of this treatment (Chromating) is the fact that the chromates are quite toxic and their use in systems which are eventually diverted into natural water sources has been substantially curtailed by Federal and Local regulations.
Other corrosion resistance filming/chromating compositions which contain one or both of the chromates or condensed phosphates are similarly subject to the disadvantages set forth above.
A further disadvantage of these and other prior art metallic conversion systems is their inability to function under a variety of adverse environmental conditions. Some metallic conversion compositions are further limited in that they can be utilized with only a few applicable types of metals.
Different compounds/components act as and/or provide corrosion resistance for different metals. Often, aqueous systems are made up of more than one metal. Therefore, to assure maximum metallic conversion resistance, it is beneficial to employ a conversion corrosion resistance composition which will protect a variety of metals and preferably be effective under diverse operating conditions. Thus sodium mercaptobenzothiazole and/or benzothiazole are often utilized to impart surface corrosion resistances to copper, sodium dichromate for mild steel, and sodium borate together with sodium nitrate to impart surface corrosion resistance to cast Iron. However, it is seen that merely combining known individual metal corrosion retardants/inhibitors together, will not give adequate protection under a variety of conditions without the disadvantages set forth previously.
Applicant is also aware of U.S. Pat. No. 4,225,351 to Zuendt et al. This patent discloses a non-toxic solution designed to impart improved brightness and corrosion resistance to metal surfaces such as, for example, zinc-plated surfaces. While these general purposes are shared by the present invention, Applicant herein has discovered a different combination of constituent ingredients in much smaller proportions which is at least as effective as the Zuendt et al. composition.
Applicant also wishes to make reference to Applicant's prior U.S. Pat. No. 4,711,735 which discloses a coolant additive with corrosion inhibitive and scale preventative properties. While the additive disclosed in the prior patent is specifically intended for use in combination with a coolant with particular application to coolants used in conjunction with diesel engines, the composition disclosed in this patent application is more general in scope and is applicable to coat and protect metallic surfaces whether or not a coolant is being employed and whether or not the environment consists of the cooling system of an engine.